The flow rate through the UV sterilizer has to be fairly slow for the UV sterilizer to be effective. The bigger the UV sterilizer, the faster the flow rate can be. For our 300 gallon tank, we connected our 40 Watt Rainbow Lifegard UV sterilizer to the return line from the FX-5. The FX-5 drives the water through the UV sterilizer and back into the tank. Our UV sterilizer has perform flawlessly for us--in very short order it cleared up our green water problem. (with large picture windows and skylights, our tank is in sunlight all day long). The flowrate of the FX-5 is still less than the maximum recommended flowrate through our 40 Watt UV sterilizer.
Whether you can connect your UV sterilizer directly to your FX-5 will depend on the size of your UV sterilizer. BTW, the 40 Watt Lifegard UV sterilizer is very tall (36"), so cannot be easily housed. It sits on the floor behind our tank, along with the FX-5 and our wet/dry filter. The beauty of the Lifegard is that the bulb does not have to be replaced as frequently as other UV sterilizers.